The enzymes catalyzing the inteconversion of proline and pyrroline-5-carboxylate provide a mechanism for the intercompartmental, intercellular, and interorgan transfer of redox potential. Mediated by the transfer of redox potential, PC regulates PRPP and nucleotide production thereby linking amino acid and nucleotide metabolism. The catalytic stimulation of nucleotide synthesis by the proline cycle suggests that PC may be a second messenger intracellularly or a "primitive hormone" intercellularly.